The present invention relates generally to supplying energy to equipment during interruption of the normal energy supply. More particularly, the present invention involves supplying, on a short-term basis, electrical energy which has been stored in a capacitor bank to equipment when the normal AC (alternating current) source of energy fails.
Much equipment in commercial and industrial use today operates on commercially available AC (alternating current) power. Failures of this supply of AC power do occur and it is often desired to keep certain devices operating, even during such a failure. To this end the use of standby power supplies are well known.
Perhaps the simplest standby power supply available is the storage battery. It is known to monitor the AC power supply and when it fails the storage battery is connected to the equipment. For many kinds of equipment this is adequate. However, for other equipment such as computer and certain telephone equipment the delay involved in switching over to a standby battery (even a delay of one second, or less) can be disasterous; the data stored in the computer can be lost and telephone connections can be interrupted. Storage batteries also have the drawbacks that they are relatively expensive, they require maintenance, and they occupy relatively large amounts of space.
It is also known to use large capacitors in a power supply. These capacitors may provide a filtering effect, but they also store energy that can be supplied to the equipment in the event of a failure of the normal power source. The following U.S. patents describe this general technique: U.S. Pat. No. 4,124,103 dated Nov. 7, 1978 by P. R. Otto et al; U.S. Pat. No. 4,247,913 dated Jan. 27, 1981 by T. K. Hiniker et al; and U.S. Pat. No. 4,335,434 dated June 15, 1982 by K. Baumann et al. These patents all have in common, the concept of storing energy in capacitors at the normal operating voltage of the system. As a result, if more energy is to be stored, then capacitors having a larger capacity must be employed. This is disadvantageous because larger capacitors cost more and they occupy more space. Additionally, once the capacitors begin to discharge, the voltage across them drops and it soon becomes too low to power its load.
The article entitled "Specifying Uninterruptible Power Systems" by R. Caprigno et al in Digital Design, March 1983, pages 88 to 91 describes generally the principles underlying uninterruptible power systems.